Acyl-acyl carrier proteins (acyl ACP's), as the primary substrates for enzymes of the fatty acid synthetase complex, lie at the heart of a system responsible for fatty acid biosynthesis and ultimately the acyl chain distribution found in membrane lipids. Yet, little is known about the way in which ACP binds growing fatty acyl chains, the way in which the thioester bond at the growing end of the chain is exposed to various enzymes of the synthetase complex, or the way in which exposure may vary with environmental factors. Our specific aims include several steps toward understanding the interactions between acyl chains and ACP of E. Coli. We plan to structurally characterize ACP and acyl ACP's using recently developed 1H two-dimensional NMR methods and supplementary methods relying on specific labeling of acyl chains with 19F or 13C. We also plan to characterize the motions of ACP and acyl chains bound to ACP using primarily 13C spin relaxation and 1H amide exchange. Changes of structure and dynamics with environmental factors such as pH, temperature and ionic strength will be correlated with enzyme activities to provide a better understanding of control of this important biosynthetic system.